South Africa's May 6 white election could mark the start of a shake-up in how the country is run, and by whom. To be sure, some 10 days before the campaign starting gun goes off, the incumbent National Party seems to run no risk of losing power. But analysts feel it is almost equally certain that Pieter W. Botha, the National Party (NP) leader and South African President, is contesting his last election.

For months now, the South African press has been rife with speculation over who may ultimately succeed Mr. Botha. And white politics, for years as immutable as Cape Town's Table Mountain, are showing sudden signs of flux.

Even some political leaders who have long dismissed South Africa's white elections as irrelevant are quietly reassessing their views. Some major antigovernment figures such as Archbishop Desmond Tutu and clergyman Allan Boesak have stuck to their argument that the only responsible approach to the election is to boycott it.

But some members of the major extraparliamentary opposition force, the multiracial United Democratic Front, are coupling their boycott of the election with efforts to influence the whites who do participate. The anti-apartheid women's group, Black Sash, has paired its decision to boycott with a similar exercise in bridge-building.

``We will use the election [campaign] to reach white people who are disillusioned with the status quo ... whether they vote or not,'' says Black Sash veteran Sheena Duncan.

At the least, the election results are expected to affect who next rules the NP. The NP rode the doctrine of race segregation, known as apartheid, to power in 1948 and has ruled ever since.

At present, there are two major contenders: Cape Province NP chief Chris Heunis, a main architect of Botha-era efforts to bring blacks into some form of ``power sharing'' with South Africa's dominant white minority, and the reputedly more conservative Transvaal Province NP leader, Frederik de Klerk.

But Mr. De Klerk and his Transvaal colleagues are under assault by the vehemently anti-reform Conservative Party, which broke with the NP in '82. Mr. Heunis faces a surprise challenge from Denis Worrall, the recently retired ambassador to London. Mr. Worrall is one of several NP veterans running as independents on a platform of more far-reaching race reforms.

Should the NP take big losses in the Transvaal, it would hurt De Klerk's succession chances. Likewise, Heunis's position would be hurt if he were unseated by Worrall. Should both occur, the nod could go to a dark-horse contender. The most frequently mentioned is Foreign Minister Roelof Botha (no relation to the President), long the most articulate and outspoken official voice for bringing blacks into national government.

The opposition parties on both left and right hope the May election will do far more than rejigger the NP succession race.

Neither side predicts the defeat of the ruling party. But both claim that a strong showing this time would so traumatize the NP that it could not possibly win an outright majority in 1989. (The abnormally long six-year gap between the previous vote and this one requires that another election be held by 1989.)

At present, to local analysts' surprise, the liberals' challenge seems the stronger. When President Botha first aired the idea of an early election last year, the idea was to parlay his state-of-emergency crackdown on black unrest and widespread South African resentment of Western sanctions into a reinvigorated mandate from white voters.

Two things have since changed the campaign picture:

Reduced violence and a slight economic rebound have dented whites' sense of siege, giving them the luxury to weigh faster retreat from apartheid.

The Conservative Party finds itself spending less time whipping up opposition to the NP than bickering with the smaller far-right Herstigte Nasionale Party (Reconstituted National Party) over an alliance to avoid splitting rightist strength.

The liberal opposition, too, is having trouble starting its engines. The Progressive Federal Party, which holds 26 opposition seats, is moving convincingly to position itself for a united ``reform'' push in 1989 by leaving liberal-leaning NP candidates and ``independents'' unopposed. But these putative allies have responded cautiously.

NP reformers have been leery of hinting at an eventual split with the party. The three ex-NP independents issued a manifesto steering a middle line between the Progressives' strongly anti-apartheid stand and the NP stress on a need to combat black radicalism.

The manifesto couples a call for a ``freeing'' of black political activity with support for a continued state of emergency ``to secure stability during the transition.'' The independents are reluctant to appear to endorse dealings with the outlawed African National Congress. But Wynand Malan, the first breakaway, suggests the ANC's political platform could be pressed nonviolently by others in talks on a new political equation.

Pre-campaign polls have offered a murky picture of what to expect. One, commissioned by a pro-NP newspaper, suggests the incumbents will suffer little at the hands of either left or right. The NP now holds 127 seats of 175 in the white legislature.

But a Progressive-commissioned survey finds 35 percent of respondents favor the NP approach to reform, while an equal slice wants a new liberal alliance of the sort the Progressives hope will emerge from the May 6 vote.

Much will depend on what happens in the months ahead - on the unrest front, on sanctions moves, and on the hustings.

This report was written in conformity with South Africa's press restrictions.